Duplicate items
Equip a Scroll that you have a multiple of (for example, Minor Heal Other x10). If you have ten scrolls you will get ten items. Swing a weapon. During the swing, open your inventory and select the scroll you equipped twice. You will not get a message. Drop the item to be duplicated. You can only duplicate things that you have less than the amount of scrolls you equipped. For example, with ten scrolls, you cannot duplicate fifteen potions. There is a way to get around this. Drop a few of the items you want to duplicate before you start this glitch so you have less than the amount of scrolls that you have. You can then pick them up afterward.

Glitch: Duplicating Items after downloading Shivering Isles
This glitch allows you to duplicate items by using scrolls:

Step 1: Go to your inventory.
Step 2: Equip any scroll that you have multiple copies of (ex. 5 "Superior Convalescence").
Step 3: Once the scrolls have been equipped, press A again while still in the inventory.
Step 4: Drop the item you wish to duplicate (ex. Grand Soul Gem) - you must have less items than scrolls or it doesn't work.

You will now have the same number of items(ex. Grand Soul Gems) as you have scrolls!